The preparation of metal-dispersed carbon paste electrodes, based on mixing an organic binder with metalized graphite, Is described. Such electrodes combine the efficient electrocatalytlc activity of metal microparticles with the attractive properties of carbon paste matrices. The ability to catalyze the slow electrode reactions of hydrogen or organic peroxides, hydrazine compounds, ascorbic acid, and dlhydronlcotlnamlde adenine dHiudeotlde Is Illustrated. Carbon pastes doped with different metals (Pt, Pd, and Ru) exhibit different electrocatalytlc activities, background current contributions, and hence analytical performances. Co immobilization of an oxidase enzyme allows coupling of a blocatalytlc reaction with the electrocatalytlc detection of the liberated peroxide. The decrease In operating potential greatly benefits the amperometric monitoring of flowing streams. The attractive coupling of metal catalytic centers with carbon paste formulations should be very valuable for routine qualitative work.

The sensitivin and stability of carbon paste enzyme electrodes are greatly enhanced through the incorporation of fumed silica within the paste matrix. The improved performance is attributed to the adsorptive and electrocatalytic properties of the silica nanoparticles. The electrocatal\-tic action results in a substantial lowering of the overvoltage for both the oxidation and reduction of the enqmatically liberated peroxide species. Highly selective, low-potential, biosensing of glucose is thus accomplished in the presence of coexisting acetaminophen. ascorbic acid, or uric acid. The enhanced performance accrued from the presence of silica is also illustrated for mediator (dimethyl ferrocene)-containing pastes. High enzymatic activin is maintained over prolonged periods. The silica modification strategy provides an effective, fast, and inexpensive route for immobilizing enz).mes within carbon paste matrices.

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Biocatalytic accumulation. followed by chronoamperometric detection, is shown to greatly enhance the sensitivity of mediatorless biosensing of hydrogen peroxide at horseradish peroxidase (HW)/carbon paste bioelectrodes. The mixed carbon paste, with its close proximity between the biocatalytic and graphite sensing sites, promotes the mediatorless accumulation of the oxidized form of HRP. Hence, a 2&fold enhancement of the response for 10 p M hydrogen peroxide is obtained following 3-minute preconcentration. The relative standard deviation (surface-to-surface) at the 10 p M level is 3.3%. The effect of experimental variables, such as the preconcentration time, measuring potential, or paste composition, are explored. Analogous observations are reported for tissue (horseradish root)-based carbon pastes, for mediated HRP detection, and for monitoring of organic peroxides. Future prospects of the bioamplification approach are discussed.

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